This invention relates to cathode sputter deposition and more particularly to a method and apparatus for overcoming the adverse effects on deposited thin film uniformity from wafer to wafer due to changes in sputtering target geometry as a result of the utilization of target material.
In sputter deposition processes, substrates are placed in a processing chamber adjacent to a sputtering cathode target, which serves as a source of coating material. The pressure in the processing chamber, which is usually filled with an inert gas such as argon, is then reduced to a near vacuum, and a negative voltage is applied to the target. The negatively charged target emits electrons, which strike and ionize atoms of the gas to produce a plasma discharge. Often the plasma is intensified and confined over the target surface by the application of a magnetic field generated by magnets, which are usually placed behind or around the periphery of the target. The large quantities of positive ions from the plasma that are produced in the sparse gas within the chamber are attracted to the negatively charged target, bombarding its surface and thereby dislodging atoms or small particles of the material of which the target is made from the surface of the target. The atoms or particles move across the space in front of the target until they strike the surface such as the surface of a semiconductor wafer or other substrate disposed, for example, in a plane parallel to the surface of the target, where they adhere to the substrate surface and form a thin film or coating layer thereon.
A primary consideration in designing a sputter deposition process has long been to achieve a specified degree of uniformity in the thickness of the resultant film being deposited on the substrate. In semiconductor wafer manufacturing processes, for example, such uniformities in the area of +/xe2x88x922 to 5 percent or better are currently being demanded. Factors that influence the degree of uniformity achieved in sputter deposition include the relative sizes of the target and substrates, the configurations of field producing magnets and other factors controlling the utilization or erosion profile of the target and the sputtering target to substrate spacing.
In the prior art, the factors of target to substrate size ratio, magnet design to control target erosion profile and target to substrate spacing are designed into the sputtering target and cathode assembly of the sputtering apparatus in an effort to produce the required film thickness uniformity. For a given target material, and with other process conditions being held constant, cathode assembly design has provided an ability to deposit films to some degree of the desired uniformities with targets of limited thickness, where the erosion of the target surface over the life of the target cannot materially alter the target to substrate spacing that was the basis for the system design. With such constant geometries, those skilled in the art of sputtering system design have concentrated on the control of erosion profiles, for example by altering magnet configuration, to fine tune the cathode design to achieve the desired film uniformity.
Typical prior art semiconductor wafer sputter deposition systems have employed targets of, for example ten inches (250 mm) in diameter to apply coatings to six inch (150 mm) diameter wafers. With such applications, uniformity in film thickness was approached by configuring sputtering cathode magnets to produce a greater sputtering rate around the periphery of the target, usually outside of the six inch diameter of the wafer, to simulate the incidence of sputtered material onto the substrate from the remote regions of a sputtering target of infinite diameter, which in theory would produce a equal incidence of sputtered material on every increment of the surface of the substrate. The increased sputtering rate around the periphery of the target compensates for target size limitations and increases the uniformity of the deposited film.
Theoretically also, with the target of infinite diameter, uniformity of the deposited coating is not generally affected by target to substrate spacing, at least not by spacing variations of thirty to fifty percent where other effects, not necessary to consider here, would not be factors. However, with finite targets, increased sputtering around a peripheral area of the target causes a more deeply eroded peripheral area or annular groove to form around the rim of the target. As the target erodes, the target surface recedes from the substrate, and does so faster at the target rim than at the center of the target. The target to substrate spacing change produces substantial changes in the rate at which material is deposited in the vicinity of the rim of the substrate. However, at the center of the substrate, the deposition rate is much less affected by such changes.
With a ten inch target used for coating six inch wafers, targets having thicknesses of from one-sixteenth inch to one and one-half inches are commonly found. Typically, target-to-substrate spacing with such targets may be approximately two inches. With the thin target, the target-to-substrate spacing change experienced over the life of the target will be at most about three percent, which should have a negligible effect on the deposition uniformity on the substrate. With the thicker targets, however, the erosion of a peripheral groove can result in an increase in target-to-substrate spacing, at certain points on the target, by more than seventy percent. Such changes can result in substantial decreases in the deposition rates on the substrate, particularly in the vicinity of the substrate rim. Thus, cathodes designed to produce a desired coating uniformity on wafers early in the life of the target do not coat wafers with sufficiently uniform films late in the life of the target.
Some prior art systems have been proposed in which the deposition rate xe2x80x9croll-offxe2x80x9d or decrease over the life of the sputtering target due to the progressive erosion of the target is offset by an increase in sputtering power. Such increases in many such systems have a uniform compensating effect across the surface of the target. Thus, where the erosion rate roll-off is usually greater at the peripheral groove on the target than at the target center, the uniformity of the coating changes along with the reduction in deposition rate as the target erodes, while the loss of uniformity is retained as the sputtering power is increased. Some systems have disproportionately increased sputtering power around the peripheral groove. While such adjustment is possible where stationary electromagnets are used, in those sputtering systems where rotating permanent magnets are desired, magnet field compensation for non-uniform deposition rate roll-off is less practical and less effective. This increases cathode assembly complexity, and is difficult with one piece sputtering targets, and tends to even more greatly increase the erosion rate around the rim of the target in proportion to the area in the target center. Such schemes of compensating for erosion, however, also have effects on voltage levels, component heating and plasma shaping that have other often adverse, undesirable or troublesome effects.
Accordingly, there is a need for an effective and efficient method and apparatus for maintaining high degrees of deposited film uniformity in a sputter coating process, particularly where thick targets are employed which substantially are substantially eroded over their useful lives.
It is a primary objective of the present invention to maintain a desired sputtered film thickness distribution throughout the life of a sputtering target. A more particular objective of the present invention is to overcome changes in deposited film uniformity caused by changes in the geometry of the surface of a sputtering target as the target erodes. A specific objective of the present invention is to provide a method and apparatus for maintaining the uniformity of films deposited by a thick sputtering target, from substrate to substrate, over the lifetime of the target. It is a further objective of the present invention to provide for the automatic adjustment of conditions or parameters of sputter deposition to consistently produce uniform film thickness on substrates being coated, over the useful life of the sputtering target, without the need for manual adjustment by an operator or other operator intervention.
In accordance with the principles of the present invention, there are provided a method and an apparatus in which the relative position of the substrate in relation to the position of the sputtering target is changed to compensate for changes in target geometry, due to the consumption or erosion of the surface of the sputtering target during use, and to thereby maintain the deposition uniformity. The change in target-to-substrate spacing is preferably made automatically in response to a measurement of some parameter related the target erosion, and is made to achieve a distance that varies in a predetermined relationship or as a predetermined function of the state of erosion of the target.
According to the preferred embodiment of the present invention, the relative positions of a substrate holder and a target support on a cathode assembly are changed during the life of the target. The position changes are made to maintain a predetermined distance between the surfaces of substrates that are to be coated, when mounted on the substrate holder, and the surface of a sputtering target, supported on the cathode holder in the cathode assembly, as the target is consumed during the processing of a sequence of wafers. As a first order of approximation, the predetermined distance, as a function of target erosion, may be considered a linear function that maintains the substrate at a constant distance from the average depth of the erosion groove. However, due to effects such as self-shadowing, redeposition, and other effects, it is generally found that the function is an increasing function requiring a greater decrease in target-to-substrate spacing for each unit of erosion of the peripheral groove. Furthermore, the relationship between the relative positioning of the substrate and target and target erosion is dependent on target material, magnetron design, the absolute distance from target to substrate and other factors. Thus, either a correction table, a list of coefficients or a specific function is preferably provided, most conveniently in software or in the form of data to be input to a controller of the machine. Such data or function may be generated, for example by a sputtering apparatus and target manufacturer, by coating test wafers, one at each of a plurality of spacings at, for example, 0.05 inch increments over a range, at various times, for example 10 or more, over the life of a test target. In this way, a table or equation may be generated describing the relationship between target-to-substrate spacing and coating uniformity.
The spacing of substrates supported on a holder in a sputter coating chamber and a target of a sputtering cathode assembly in such chamber is so maintained throughout the sputtering life of the target, by continuous or periodic adjustments of the target to substrate spacing. Normally this requires reducing the distance between the target and substrate holders, but in some cases an increase in the distance may be required to maintain uniformity. The adjustments may be made during the processing of individual substrates but are preferably performed between coating operations, when changes of the wafers or substrates are being made. Such distance changes are made progressively at various intervals throughout the life of the target, whenever enough of a change in the profile of the target is anticipated to affect coating uniformity. Such changes might be made after every fifty wafers are processed, or at one hundred or more times over the life of a target. With thicker targets such as aluminum which deposit thicker films onto the substrates and accordingly erode to depths of more than an inch over their lifetimes, more adjustments are required than with less thick targets.
In the preferred embodiment of the invention, a substrate holder is positioned in a sputtering chamber at an initial distance form a sputtering cathode assembly when a target is new and uneroded so as to achieve a designed target-to-substrate spacing. As the target erodes and its surface recedes from its initial position, the substrate holder is caused to move toward the target to maintain a predetermined target-to-substrate relationship. As the target erodes more deeply, the predetermined relationship changes in accordance with the collected data to maintain coating uniformity.
In the preferred embodiment of the invention, a thick target is employed which, during its lifetime, substantially changes in shape as material from its sputtering surface is depleted. Such targets are, for the coating of semiconductor wafers, for example, frequently circular in shape and somewhat larger in diameter than the diameters of circular substrate wafers being coated. Further, such targets frequently are provided with plasma shaping and enhancing magnetron structures that cause the sputtering to occur at a greater rate from a peripheral ring or annular erosion groove, usually lying outside of the diameter of the wafers being coated, than from the circular central region of the target lying within the annular groove. This use of a peripheral erosion groove causes the surface of the target to recede from its initial position faster at the area near the target rim than at the central area. In the preferred embodiment of the invention, the target-to-substrate spacing is readjusted, as wafers are processed over the life of the target, to maintain a target-to-substrate distance between the surface of the substrate and the average sputtering surface of the target to achieve desired coating uniformity.
Further in accordance with certain preferred embodiments of the invention, there is provided a sputtering apparatus having the capability of determining the state of erosion of the target and particularly the depth of the average of the erosion groove on the target surface. Such determination may be made by direct surface position measurements, for example with one or more sensors provided in the chamber. Such sensors could include, for example, laser optical devices that use light reflected from points on the target surface and produce signals related to the position or distance of the point on the target surface to the sensor. Due to target surface irregularities that develop as a target is eroded, a scanning type laser that takes several measurements and/or logic in a computer or controller to interpret the measurements would be required.
Preferably, however, erosion measurement is made by an alternative method of measuring the power consumed by the target over each increment of time from the beginning of the life of the target and the measurements summed so as to integrate the power measurement up to the present. This produces a value representative of the total energy consumed by the target from the beginning of its life to the present. This energy data is stored and continually updated. Such total energy consumption is correlated with stored data to determine the specific erosion state of the target. This is further correlated with stored data to produce a calculated representation of the erosion profile as a function of the age of the target measured in units of consumed total energy. The measurement of energy need not be from the absolute beginning of the life of the target but may be from any beginning point over the life of the target, with data being stored relating to the use of the target or its surface condition up to the beginning point of the energy measurements.
Further in the alternative, other forms of target erosion measurement may be used, such as by counting the number of wafers coated and correlating the count with data relating the count to target usage, in a manner similar to that for the measurement of energy discussed above.
In response to such measurements of target erosion, the spacing between the target holder and substrate support is adjusted, usually by reducing such spacing to achieve or maintain a desired distance between the target surface and substrate surface, preferably by moving the substrate toward the target. The amount of adjustment is preferably based on a determination from the measurements of the change in the depth of the most deeply eroded portion of the target, particularly where that portion is a peripheral groove. Alternatively, the amount of adjustment can be keyed primarily to the amount of erosion of the peripheral region of the target, or to some other function of erosion that has been empirically or by computer modeling determined to have a known relation to film uniformity, so that the amount of adjustment can be calculated based on the determination. In the embodiment of the invention in which a target is eroded to form a peripheral erosion groove, the adjustment is made by moving the substrate somewhat less than would be required to maintain a constant distance between the substrate surface and the bottom of the peripheral groove. Preferably, the sputtering apparatus is provided with a microprocessor in its control that calculates such adjustment based on a stored table or function in memory.
In response to the calculations based on the target surface position measurements, either the target or substrate, but preferably the substrate, is moved to adjust the target-to-substrate spacing, keeping the target and substrate parallel. The movement may be achieved by providing a servo or stepper motor linked to the substrate holder, for example, by a gear or ball screw drive or other suitable drive mechanism. Preferably, a feedback sensor of some sort is employed to verify the operation of the motor. Such, sensor may be a position indicator built into the servo or stepper motor or motor driver, or such as a position decoder or resolver linked to the motor output or otherwise between the substrate and substrate support.
In an alternative embodiment, a deposition rate sensor may be employed to measure the incidence of sputtering material across the wafer surface. Such a sensor may be set up to take deposition readings between the processing of wafers so that the sensors can more easily occupy the position of the wafer. The controller of the apparatus may then be programmed to adjust the position of the substrate holder or target until uniform incidence of material is realized. In the alternative, servo or stepper motor with feedback control from such a sensor may be used.
With the method and apparatus of the present invention, the sputtered film uniformity that existed across the surfaces of wafers sputtered from the target at the beginning of the target""s life is closely maintained across wafers coated sputtered throughout and toward the end of the life of the target.